Well according to wiki which has plenty of references listed Mahayana came after Theravada. Theravada literally means school of the elders. It is based around the original teachings of the Buddha.

As for Theravada being the only direct path that leads to liberation? The Mahayana path's (especially Zen) require you to take the Bodhisattva vow which means you will be reborn over and over again until all sentient beings are saved, delaying your own pari nirvana potentially for eternity.

The Buddha did not teach the Bodhisattva vow. It was made up hundreds of years after his death.

I have some theories of why it might of been made up (such as some people clinging to life as a human and thus wanting to come back as human again and again).

Finally, I looked up Tantra and this popped up. https://en.wikipedia.org/wiki/Tantra

It comes from Hinduism/Yoga. Which means that Tibetan Buddhism due to it's geological location did borrow a lot of it's concept's from Indian religions that predate Tibetan Buddhism but are not found in the other forms of Buddhism such as Zen or Theravada.

Also show me where the Buddha taught the Mahayana sutras. From what I have read they were made up hundreds of years after his death and that there are no sources of the Buddha teaching these supposed "secret teachings" if that is what you are referring the Mahayana sutras as.